Various constructions of guide systems of the mentioned kind are known, as described below.
Sleeves mounted co-axially about the optical axis and movable within one another and in the housing and each containing a zoom component may form the guide system of a zoom. As a rule there is substantial lateral clearance between the zoom components and the housing because of the total diametric tolerances. Moreover, in the case of sleeves with a short axial length, there is a risk of tilting the individual zoom components particularly when the sleeves are pushed into one another. On the other hand, a small diametric clearance means that the zoom components cannot move easily within the housing. After the sleeves with the zoom components have been built into the housing the lenses are no longer accessible so that it is not possible either to clean the lenses or to adjust the zoom components in the lateral or axial direction.
Another known zoom guiding system consists of guide columns arranged parallel to the optical axis on which slides move which accommodate the zoom components. One column acts as a guide column and another acts as a rotation preventer to prevent the zoom component from rotating about the optical axis. In this arrangement, narrow diametric tolerances have to be maintained in the columns and slides to avoid any play. Any flexing of the columns leads to guiding errors. In addition, narrow positional tolerances of the columns also have to be maintained. The slides which hold the zoom components have a complex geometry in this solution. As a result of the tolerance totals resulting from the complex structure for the position of the axis of the zoom components (optical axis), the zoom components have to be laterally adjustable.
Finally, a coaxially constructed guide system is known having disk-shaped slides which accommodate zoom lenses and which are supported for lateral positioning within the diameter of the inner wall of the housing. The actual guiding of these slides and precautions against tilting are provided by sets of three bolts arranged in the slide which pass through longitudinal slots in the housing and abut on a triple curve provided about the housing. This solution is also unable to prevent possible tilting of the zoom components as the result of unavoidable tolerances in the triple arrangement in spite of a more expensive circumferential curve. As a result of the diametric tolerances needed to permit movement there is a risk of lateral slippage of the zoom components which can only be counteracted by complex measures. Moreover, these disadvantages are a result of the unsatisfactory nature of the lenses, namely the impossibility of axial or lateral adjustment of the zoom components and the impossibility of cleaning the lenses once they have been installed.
U.S. Pat. No. 5,675,442 discloses a guide system for microscope lenses in which two guide rods guide one or more lens holders along an optical axis. A slide of a lens holder fits around one of the two guide rods in interlocking engagement and thus acts as a guide column. The other guide rod acts as a rotation preventer, the lens holder partially surrounding the guide rod. For this purpose the lens holder has, on the relevant side, a unshaped recess in which the guide rod engages. In order to avoid possible displacement within this recess a magnet is provided on one side of the recess which exerts a magnetic attracting force on the associated magnetisable guide rod so that the latter always abuts on the side of the U-shaped recess where the magnet is located.
The disadvantages connected with this solution have already been described above in connection with the second solution described.